1. Field of the Invention
The invention relates to a system and a method for neural stimulation of a patient's hearing, such as by cochlea stimulation.
2. Description of Related Art
The sense of hearing in human beings involves the use of hair cells in the cochlea that convert or transduce acoustic signals into auditory nerve impulses. Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensori-neural. Conductive hearing loss occurs when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded. These sound pathways may be impeded, for example, by damage to the auditory ossicles. Conductive hearing loss may often be overcome through the use of conventional hearing aids that amplify sound so that acoustic signals can reach the hair cells within the cochlea. Some types of conductive hearing loss may also be treated by surgical procedures.
Sensori-neural hearing loss, on the other hand, is caused by the absence or destruction of the hair cells in the cochlea which are needed to transduce acoustic signals into auditory nerve impulses. People who suffer from sensori-neural hearing loss may be unable to derive significant benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus is. This is because the mechanism for transducing sound energy into auditory nerve impulses has been damaged. Thus, in the absence of properly functioning hair cells, auditory nerve impulses cannot be generated directly from sounds.
To overcome sensori-neural hearing loss, numerous auditory prosthesis systems (e.g., cochlear implant (CI) systems) have been developed. Auditory prosthesis systems bypass the hair cells in the cochlea by presenting electrical stimulation directly to the auditory nerve fibers. Direct stimulation of the auditory nerve fibers leads to the perception of sound in the brain and at least partial restoration of hearing function.
To facilitate direct stimulation of the auditory nerve fibers, a lead having an array of electrodes disposed thereon may be implanted in the cochlea of a patient. The electrodes form a number of stimulation channels through which electrical stimulation pulses may be applied directly to auditory nerves within the cochlea. An audio signal may then be presented to the patient by translating the audio signal into a number of electrical stimulation pulses and applying the stimulation pulses directly to the auditory nerve within the cochlea via one or more of the electrodes.
Typically, the audio signal, which usually is captured by a microphone, is divided into a plurality of analysis channels, each containing a frequency domain signal representative of a distinct frequency portion of the audio signal, wherein the frequency domain signal in each analysis channel may undergo signal processing, such as by applying channel-specific gain to the signals. The processed frequency domain signals are used for generating certain stimulation parameters according to which the stimulation signals in each stimulation channel is generated. The analysis channels are linked to the stimulation channels via channel mapping. The number of stimulation channels may correspond to the number of analysis channels, or there may be more stimulation channels than analysis channels, or there may be more analysis channels than stimulation channels. Various stimulation strategies are used, such as current steering stimulation (in order to stimulate a stimulation site located in between areas associated with two or more electrodes) and N-of-M stimulation (wherein stimulation current is only applied to N of M total stimulation channels during a particular stimulation frame).
An example for such a CI system with electrical cochlea stimulation is described in International Patent Application Publication WO 2011/032021 A1 and corresponding U.S. Pat. No. 8,422,706.
Patients, who are precluded from using a cochlear implant due to an illness or injury that has damaged the pateint's cochlea or auditory nerve, may be provided with an auditory brainstem implant or an auditory midbrain implant. Such devices use similar technology as a cochlear implant, but instead of electrical stimulation being used to stimulate the cochlea, it is used to stimulate the brainstem or midbrain of the recipient.
With CI systems, speech intelligibility is restored at a great inter-individual variability, i.e., some CI patients achieve open speech intelligibility, whereas other CI patients are not able to understand speech without visual assistance, such as lip reading or sign language. In particular, speech perception is difficult for many auditory prosthesis users when using a phone, since audio signal bandwidth of telephone systems is limited to a frequency range of about 300 to 3400 Hz, while speech audio signals have a frequency range of about 100 to 8000 Hz. While normal hearing persons usually are able to understand such band limited speech, hearing impaired persons often have problems.
An established approach for enhancing speech intelligibility during phone use of hearing impaired persons is to provide the hearing device with a telecoil (T-coil) which records the inductive audio signal produced by the loudspeaker of the telephone device and presents this audio signal as input to the hearing device, whereby an enhanced signal to noise ratio can be achieved.
Due to the limited spectral resolution of neural stimulation auditory prosthesis devices, such as CI systems, auditory brain stem implants or auditory mid-brain implants, reduced speech intelligibility due to bandwidth limitation of the input audio signal, such as present in a telephone audio signal, is particularly severe.
The article “A Phone-Assistive Device Based on Bluetooth Technology for Cochlea Implant Users” by H. Qian et al., IEEE Transactions on Neural Systems and Rehabilitation Engineering, 2003, pages 282 to 287, proposes a wireless phone adapter based on Bluetooth technology which can be used to route the audio signal from a telephone device directly to a hearing aid or CI processor.
The article “Frequency-to-electrode allocation and speech reception with cochlear implants” by C. McKay et al., J. Acoust. Soc. Am. 111 (2), 2002, pages 1036 to 1044, relates to speech recognition tests on CI systems, wherein the frequency-to-electrode allocation was varied.
The article “The effect of short-term training for spectrally mismatched noise-band speech” by Q.-J. Fu et al., J. Acoust. Soc. Am. 113 (2), 2003, pages 1065 to 1072, relates to a study wherein acoustic information was spectrally distorted by shifting speech information from one frequency region to another, with the audio signals being presented to normal hearing persons.
The article “Frequency-place compression and expansion in cochlear implant listeners”, D. Baskent et al., J. Acoust. Soc. Am. 116 (5), 2004, pages 3130 to 3140, relates to a study wherein effects of frequency-place compression and expansion on speech perception by CI users were investigated.
European Patent Application EP 2 375 782 A1 and corresponding U.S. Pat. No. 8,949,113 relate to a signal processing in instruments, wherein it is mentioned that frequency compression or expansion may be used for reducing bandwidth requirements for an audio transmission channel such as a telephone standard channel.
U.S. Pat. No. 8,098,859 B2 relates to a CI system comprising a frequency upward-shifting processor and a formant upward-shifting processor for shifting low frequency audio signals into a higher frequency range, with such system being applicable to patients suffering from low frequency hearing loss.
International Patent Application Publication WO 2008/154706 A1 and corresponding U.S. Pat. No. 8,605,923 relate to a CI system having several audio signal processing modes which are selected according to the result of an auditory scheme analysis.
U.S. Pat. No. 7,711,133 B2 relates to a CI system wherein the frequency resolution of the filter bank is higher for certain frequency ranges, such as the frequencies most relevant for speech recognition.